In connecting an electric generator, transformer or similar equipment to an associated electrical system, a circuit breaker connecting the generator and the electrical system is closed after selected operating conditions of the generator and the electrical system are satisfied. The connection must occur when the two systems are synchronous in their respective operation; the systems can be damaged when the breaker closes when the systems are not synchronous. The general arrangement of two connecting systems is shown in FIG. 1, which includes a generator 10, an electrical system 12, a transformer 14 and a connecting circuit breaker 16. Circuit breakers 18, 20 and 22 are associated with the electrical system 12.
Synchronous power generators are started when they are electrically isolated from the electrical system. Connecting such a generator to a power system is dynamic and requires the coordinated operation of electrical and mechanical functions as well as human action. The generator must be connected to the power system with minimal power surges or swings. This is accomplished by closing the connecting breaker (16 in FIG. 1) when the generator substantially matches the power system in voltage magnitude, phase angle and frequency. Typically, there will not be an exact match and some power will flow into or out of the generator to force it into synchronization with the power system. If that power is excessive because of a poor match, severe damage to the generator or the power system can result.
The generator 10, once started, will begin to increase in speed prior to connection with the electrical system 12, and once the speed is relatively near (typically slightly greater than) the synchronous speed appropriate for the electrical system, closure of the system circuit breaker 16 will be initiated to connect the generator to the electrical power system. Various ways have been used to determine the best point in time for initiating closure of the circuit breaker, including monitoring of the generator operating angle, the speed (frequency) of the generator and the voltage magnitudes of the generator and the power system. Typically, the generator slip frequency and the closing time for the circuit breaker are important variables in determining the time at which to initiate the closing of the system circuit breaker. The slip frequency is the difference in frequency between the generator voltage and the system voltage.
Once circuit breaker closure is initiated, however, there is no way to stop the circuit breaker from closing, even though closing may have been initiated at the wrong time and/or the closing may be too late. Attempting to reverse a partially closed breaker could result in extreme failure of the breaker.
When the circuit breaker begins to close, it will either close within the expected and acceptable time limits, or it will close slower than expected but still within an acceptable time for the power system and the generator, or lastly, the circuit breaker will not close at all, or at a slower rate than is acceptable. Delay in closing can be due to a variety of causes, including electrical problems or mechanical problems, such as corrosion, degraded lubrication, etc.
There is thus a range of angles between the voltage and the generator voltage which is acceptable at the time of actual closure of the circuit breaker. Generally, for a particular angle, as long as the circuit breaker is able to physically close without causing severe damage to the generator or electrical system, that particular difference in angle is acceptable. Existing circuit breaker controls typically take into account the estimated circuit breaker closing time and initiate closing of the breaker at such a time that when the circuit breaker actually closes, the generator and system voltage angles will be within a small difference angle. If the circuit breaker operates slower than expected, but not too slowly, it still may actually close within an acceptable angle such that damage does not result. However, if a circuit breaker operates so slowly that the generator and the system voltage angles are no longer within an acceptable range of difference, damage can result to the system or the generator. As indicated above, however, even if it is known that the breaker closing is going to take too long, the breaker could not itself be stopped from closing.
FIG. 2 is a diagram illustrating the voltage angle relationship between the generator and the system. The solid line 26 represents for illustration a system angle (at 12 o'clock), while solid line 28 represents a generator voltage angle which is rotating clockwise, due to a slightly higher frequency. The position of line 28 is for illustration only. Area 30 shown between the dotted lines 31--31 represents a "desired" voltage angle range, i.e. .+-.3 degrees. Area 32 between the solid lines 33--33 represents an angle difference which is beyond the desired angle difference but which is acceptable for proper operation. This could be as much as .+-.70 degrees. A more typical value, depending upon the particular application, will be .+-.10-15 degrees. If the circuit breaker, however, is even slower, it may actually close beyond lines 36--36. In this situation, the voltage angles are so different that severe system damage will result.
As indicated above, the circuit breaker takes a finite time to physically close after closure is initiated. In order to accommodate this finite time, the operator (or the automatic control system) who is initiating the closure must actually initiate closure of the breaker prior to its reaching the desired angular difference region. The actual value of generator slip and the circuit breaker close time determines the point at which the circuit breaker closure is initiated.
There are various known methods to accomplish the synchronizing process which takes into account various closing issues. The closing procedures, however, do rely on the breaker closing in a specified time. To protect against a slow breaker, a timer has been used. If the breaker does not actually close within that predetermined time, referring again to FIG. 1, breakers 18, 20 and 22 will all be tripped, thus clearing the system bus serviced by the generator. This clearing of the bus is an extreme situation and should be avoided if possible, although it is better than permitting the system to be damaged. Further, the timer is typically set very conservatively, i.e. a relatively short time, to ensure against system damage. It is thus possible that the bus circuit breakers could be tripped in a particular situation even though the system circuit breaker would have in fact closed in time.
Hence, it is desirable to provide a control circuit which monitors the actual operating characteristics of the generator and the power system such that the bus circuit breakers will trip substantially only when the system circuit breaker is in fact going to close late. Such a system would thus be both secure yet efficient for generator/power system synchronization.